Fluid-solid reaction systems, such as gas-solids reaction systems, often require the solids to be retained in early stages of the reaction system while the vapor product, essentially free of solids, is processed in downstream equipment. It is desirable in these systems that the solids be as completely removed as possible from the vapor before transferring the vapor to the downstream equipment. High solids retention in the early stages of the reaction system is desirable in cases in which the solids may contaminate the vapor product or downstream vapor process handling systems, and/or increase the capital and operating costs of downstream particulate capture devices such as wet gas scrubbers, electrostatic precipitators, or filters. Additionally, in reaction systems that use small particle catalysts, the loss of catalyst particles during operation means that additional catalyst has to be added during operation to make up for the catalyst loss. Particularly in cases where the cost of catalyst is high, even marginal improvements in solid particle retention can lead to substantial reductions in operating costs. Therefore, improvements in high efficiency solids/vapor separation systems are of particular interest.
One method for separating solids from a gas-solids flow is to pass the gas-solids flow through one or more cyclone separators. For example, cyclone separators are conventionally used to separate particles from gas-solids flows in fluidized bed reactor systems such as FCC reactors and oxygenate-to-olefin reactors. In these systems, cyclone separators can be arranged in “stages” so that the lower density or gas output of a first cyclone separator stage becomes the input for a second cyclone separator stage.
Although the cyclone separators can be arranged in stages to improve efficiency, in practice the number of stages is limited by constraints on the input and output flows of the cyclones. Once the majority of solids have been removed from a gas flow, the remaining solids in the flow may not be sufficient to allow a conventional cyclone separator to operate at full efficiency. In particular, it is difficult to design a multistage cyclone separator, as the amount of solids in the input flow for any cyclone stages after the first cyclone stage is often too low for fully efficient operation of a conventional cyclone. Due to the low rate of solid catalyst particle flow, in multistage cyclone separators the diameter of the final cyclone stage dipleg can often be ½ inch or less. At such a dipleg diameter, the dipleg is prone to catalyst bridging or compaction. Catalyst bridging and/or compaction prevents outflow of catalyst from the cyclone stage and therefore causes poor separation efficiency. However, if a larger dipleg diameter is used, the low rate of solid catalyst flow may not be sufficient to seal the dipleg. If the dipleg does not seal properly, gas can flow back into the cyclone through the dipleg, which also reduces the separation efficiency.
U.S. Pat. No. 6,533,844 to Hiltunen et al. describes a two-stage cyclone separator for use in fluidized bed reactor systems. In Hiltunen et al., the first and second cyclone separator stages are arranged concentrically so that the second cyclone separator stage is contained within the first cyclone separator stage. Based on this configuration, the first cyclone has a roughly annular processing volume. Similarly, the dipleg (higher density output) of the first cyclone has a roughly annular volume. The low density or gas output of the first cyclone is fed into the second, inner cyclone as an input flow through one or more ports in the second cyclone. The concentric arrangement of cyclones reduces the space required to house the cyclones. However, the fluid and particle transport between the cyclones is otherwise similar to conventional multistage cyclone arrangements.
What is needed is an improved process and/or apparatus for removing solid particles in gas-solids reactors, such as in reaction systems that use molecular sieve type catalysts. Preferred processes would include those that provide for a higher retention rate of solid particles, including particles of less than 50 μm in size, and those that have minimal or no impact on the efficiency of the reaction being carried out in the reactor.